Communications and broadcast satellites, and other communications arrangements, must transmit electromagnetic radiation at relatively high frequencies, on the order of 2 to 20 Ghz., and at power in excess of 10 watts. Such spacecraft include a plurality of radio-frequency (RF) power amplifiers, which generate the power required for communications transmissions. The solar panels, batteries, and power regulators of the spacecraft provide the electrical power to operate the amplifiers. In general, solid-state power RF devices are preferred to vacuum tubes for spacecraft use because of their light weight, ruggedness, and long life. For communication spacecraft use, light weight, reliability, and power conversion efficiency (efficiency in converting DC power to radio-frequency or RF power) are all important considerations. However, solid-state devices at the current state of the art do not approach the conversion efficiency of travelling-wave amplifier tubes for RF power generation. This provides the travelling-wave tube with a distinct advantage over solid-state devices for a spacecraft, where the available electrical power is limited. Also, travelling-wave tubes are available which are sufficiently rugged to withstand spacecraft launch forces, and which have a lifetime which, although finite, is greater than the expected lifetime of a spacecraft. Travelling-wave tubes have the advantage of greater amplification gain than typical solid-state devices, and are relatively broadband. For this reason, travelling-wave tubes currently dominate the RF power applications of communications spacecraft. Each tube is generally associated with one communication channel, and the spacecraft may carry as many as a few tens of travelling-wave tubes, thereby providing a few tens of broadband communication channels.
Travelling-wave amplifier tubes have certain disadvantages by comparison with solid-state amplification devices, notably in that the power-supply voltages are in the range of thousands of volts, which poses voltage breakdown problems in the components of the power supply, and also creates the potential for corona or current leakage in the vacuum of space. The travelling-wave amplifier tube includes a slow-wave structure in the form of a helix, which carries RF signal. The helix is made from thin wire because of characteristic impedance considerations. Voltage breakdown inside the travelling-wave tube may result in an arc between the anode or cathode and the helix wire, which causes direct current to flow in the helix wire. The helix wire heats in response to the flow of the arc current, and if such current is prolonged, melts the helix wire, thereby resulting in destruction of the tube.
The cost of fabricating and launching a communication spacecraft is many tens of millions of dollars, and each communication channel of the spacecraft may have a rental value in the range of one million dollars per month. It is very important to avoid destruction of travelling-wave tubes on the spacecraft, to avoid the loss of the associated communications channels and the consequent loss of revenue, and, in the limit, to avoid the need for fabrication and launch of a replacement spacecraft due to excessive reduction in the number of available channels, before the spacecraft becomes inoperative for lack of attitude control and stationkeeping propellant.
Conventionally, each travelling-wave tube in a spacecraft is fitted with its own electronic power conditioner (EPC), which produces direct voltages (often referred to as DC) for operating the tube. The EPC is arranged to shut down or de-energize in the event of an internal arc to the helix in the tube, so that an arc cannot cause current flow for a period sufficient to melt the helix wire. For this purpose, helix current is monitored by way of the helix voltage regulator in relation to ground. Under ordinary operating conditions, the electron beam travelling from the cathode of the travelling-wave tube to the collectors is magnetically focussed, so that it travels through the center of the helix, without causing any significant current in the helix. Thus, the helix current is near zero under normal operating conditions, and the presence of an arc can be established by monitoring the helix current, and enabling the shut-down protection whenever the helix current exceeds a predetermined threshold. The threshold may be selected to be, for example, three or four milliamperes (mA).
As mentioned above, the DC-to-RF conversion efficiency of the travelling-wave tube is one of its major advantages over solid-state devices for use on spacecraft. The manufacturers of travelling-wave tubes have, over time, generated new types of tubes, some with improved DC-to-RF conversion efficiency.
It has been found that at least some of the travelling-wave tube types having the highest DC-to-RF conversion efficiency also tend to trip their helix overcurrent protectors more often than less-efficient tubes. In the context of communication spacecraft, this is very undesirable, because overcurrent tripping results in shut-down of the associated travelling-wave tube, which in turn shuts down the communication channel with which it is associated. If it were possible to restart the travelling-wave tube immediately after the shut-down, the increased incidence of shut-downs might be tolerable. However, once a travelling-wave tube is shut down, it takes several minutes before the tube can be reactivated. This is because the manufacturer of the tube specifies a warm-up period; the warm-up period includes a one-minute ramp-up of the filament voltage, followed by a four-minute delay before application of high voltage, to prevent excessive thermal stress. Thus, the more frequent tripping of the travelling-wave tube power supply protection due to helix overcurrent may result in extended periods during which the communication channel is out of service. Such a situation is intolerable to those using the channels, and to the spacecraft operators who provide the channels.
It has been discovered that, in those high-conversion-efficiency travelling-wave tubes which suffer from frequent shut-downs, the normal operating point of the travelling-wave tube, at which the electron beam is focussed, the helix current is low, and the gain and output power is high, is within a very few decibels (dB) of tube saturation, at which the electron beam becomes defocussed, intercepts the helix, and results in helix current flow. For example, one type of tube has been found to transition from normal operation at full power to an overdriven condition in as little as two dB of input RF drive.
The signal paths of satellite communications channels are controlled by sophisticated systems including automatic gain and signal level controls, which tend to maintain the system operating levels under a wide variety of conditions. However, the communication channel path includes uplinks and downlinks between the ground station and the spacecraft, and both are subject to path loss variations due to random weather events, which may also affect the signal polarization. The combination of the path loss and polarization variations can result in relatively sudden signal level changes in the uplink signal received at the spacecraft. Notwithstanding the automatic level and other controls, the travelling-wave tube may experience signal level changes at its RF input terminal or port which are sufficiently large to move its operating point by one or two dB, and into the overdrive region. This, in turn, could result in shutdown of the channel due to the sensing of excessive helix current.
The use of higher-efficiency DC-to-RF converters is desirable in spacecraft communications channels, without excessive channel disruptions due to shut-down of the power supplies, while still maintaining positive protection for the conversion device.